1. Field of the Invention
The present invention relates to a hub for use in a tape casette, more particularly to a hub having an improved cylindrical surface on which a magnetic recording tape is wound.
2. Description of the Prior Art
It is well known that a hub, as shown in FIG. 1 to FIG. 3 for a tape casette is molded in a body using synthetic resin material such as polyacetal or nylon. In order to obtain a higher surface precision on the outer peripheral surface 2 on which a recording tape is to be wound, the synthetic resin material is injected into a mold through an injection port located on the mold at the position which corresponds to the upper outer annular surface 3 of the hub 1.
As illustrated in FIG. 2, crease-like hollows or wrinkles 4 are apt to appear on the outer peripheral surface 2 in the vicinity of the resin injecting gate 5. The hollows or wrinkles are formed by the flow of synthetic resin injected through an injection port located in the mold assemblies available in the prior art. When a recording tape is wound onto a hub 1 having such a crease-like hollow 4, it results in a crease-like deformation on the recording tape thereby causing distortion of the output reproduced by the recording tape.
The present inventor, as a result of studies directed to the cause of the formation of the crease-like hollow 4, has found that in the early stage of injecting the melted synthetic resin into a mold that some portions of the resin injected in the radial direction of the hub 1 flow as shown by arrows A, A' in FIG. 3. The portions of the resin flow A and A' meet one another at about the center of the axial direction of the hub 1 thereby causing a hollow to form around the central portion near the injection gate 5 on the outer peripheral surface 2.
Even though the hollow is gradually filled with the injected resin, ultimately the hollow remains in the form of one or more creases on the periphery surface 2 of the hub 1.